3.10.1 Misnesting

When a macro is called with arguments, the arguments are substituted
into the macro body and the result is checked, together with the rest of
the input file, for more macro calls. It is possible to piece together
a macro call coming partially from the macro body and partially from the
arguments. For example,

Macro definitions do not have to have balanced parentheses. By writing
an unbalanced open parenthesis in a macro body, it is possible to create
a macro call that begins inside the macro body but ends outside of it.
For example,